Connectivity Fault Management (CFM) is an IEEE standard that defines protocols and practices for Ethernet Operations Administration and Maintenance (E-OAM). CFM comprises three protocols for monitoring and troubleshooting Ethernet LANs and WANs. One of these protocols provides a connectivity check intended to permit a network administrator to determine if nodes are connected. A CFM continuity check message (CCM) is a unidirectional multicast message that is received as a heartbeat. No response to the CCM message is sent or expected, but a unicast response may acknowledge a continuity check message. The continuity protocol tests connectivity between a source node and all other nodes that are expected to be reachable by the multicast message.
E-OAM was intended to be used in a LAN environment where every source of Ethernet frames is connected to every destination of Ethernet frames via a single path in either direction. The CCM frames are sent using Layer 2 multicast and are expected to follow the single path between the devices. LANs typically use a Spanning Tree Protocol (STP) to create a single path between any two devices and that path is used for unicast and multicast packets in both forward and reverse directions. Therefore, a single packet exchange is expected to traverse every link along the path between the two devices. If a frame can travel from a source to the destination, it is assumed that the frame can also travel from the destination to the source over the same path.
OTV may be used for tunneling Ethernet frames over IP and is commercially available, for example, from Cisco Systems, Inc., San Jose, Calif., as part of the Nexus 7000 switch. OTV provides MAC-address-based routing, and allows the extension of a Layer 2 network across remote data center sites. An out-bound frame is encapsulated in an IP header and transported to the edge device within the LAN and that is coupled to the core network. The IP packet may be transported through the core network by any IP-based transport such as IP, MPLS, SONET, etc.
A LAN that is expanded into a virtual LAN over a WAN may not have a single path between source and destination nodes. Unlike LAN protocols, OTV may use different paths for unicast and multicast, which interferes with E-OAM's ability to reliably detect failures as well as to reliably diagnose failures once detected. As frames are delivered through the core network, there may be multiple routes that the frames could take to arrive at their destination. Furthermore, a frame returning from a destination back to a source may take a different path than the path taken to arrive at the destination from the source. For example, a CCM may traverse one path between a source and destination node, but another unchecked path may be used for delivering actual traffic. If the unchecked path fails, the CCM may not detect the failure.
The approaches described in this section are approaches that could be pursued, but not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated, it should not be assumed that any of the approaches described in this section qualify as prior art merely by virtue of their inclusion in this section.